Softening rubber



Patented Feb. 20, 1940 2,191,256 SOF'I'ENING RUBBER Ira Williams,Woodstowlnand Carroll Smith, Carneys Point, N. 1., assignors to E. I. duPont de Nemours & Company, Wilmington, mat, a corporation of Delaware I.No Drawing.

Application October 8, 1937,

Serial N0. 167,982

25 Claims.

This invention relates to rubber and more particularly to methods oftreating rubber to produce a more plastic product.

It is well known that rubber is rendered-more plastic when it issubjected to mechanical working in the presence of oxygen, The degree ofplastication is determined to a great extent by the temperature and thetime of milling. This action can be greatly facilitated by the use of 10small amounts of certain assistants such as the mono-aryl-hydrazines andthe thio-phenols. Such assistants, however, require time and/or hightemperatures to-produce their action. It is frequently dwirable toplasticize the rubber more 15 rapidly and particularly during mechanicalworking.

It is an object of this invention to provide an improved method forplasticizing rubber. Another object is to provide a new class of com- 20pounds for plasticizing rubber. A further object is to provide a methodfor more rapidly plasticizing rubber and new materials which willproduce such more rapid plasticizing. Still further objects are toprovide new compositions of matter and to advance the art. Still otherob- 251 jects will appear hereinafter.

These objects may be accomplished in accordance with the presentinvention by subjecting rubber, during the mixing period, to the actionof a small amount of a thio-carboxylic acidor its sulfide. The acids andtheir sulfides may be represented by the formula 35 in which R is anorganic radicle which may be aromatic, aliphatic or heterocyclic, andmay be substituted by such groups as hydroxyl, alkoxy, carboxyl, nitro,halogen and mercapto groups,

40 and X represents hydrogen or the group II II --SCR'orC--R' in which Rrepresents an organicradicle which may be the same or difierent thanR. Rand R.

are each connected to the carboxyl carbon directly by means of a carbonto carbon linkage. While the large class of thio-carboxylic acids andtheir sulfides appear to be effective for our purpose, it will generallybe preferred to employ the aromatic thio-carboxylic acids and theirsulfides, and particularly those in which R and R are aromatichydrocarbons. By the term an aromatic. thio-carboxylic acid or itssulfide, we

5 means those in which the carboxyl carbon is directly bonded to acarbon of the aromatic ring. Of these aromaticcompounds, those of thebenzene series will generally be preferred. The best results areobtained by the addition no of the plasticizing agent directly to therubber as early as possible in. the mixingoperation and in the absenceof compounding ingredients. Sulfur has been found to materially retardthe action of the thio acids and any material amount of sulfur in excessof 3% almost completely pre- 5 vents the action of small amounts of thethio acids. The efiect' of sulfur may be overcome to a certain extent bythe use 01 relatively large amounts of plasticizing agent. However, thisis unnecessarily expensive and not satisfactory.

The salts of the thio acids appear to be 'sub-' stantially ineffective.However, the salts of the thio acids may be employed in certain caseswhere the rubber contains sufflcient acidic material, either natural-ypresent or added, to release the thio acid 50 that it may exert itsaction. Accordingly, when we refer to the use of a thio acid hereinafterand in the claims, we intend to include the salts when added in thepresence of sufficient acidic material under conditions which will causethe acidic material to react with the salt to release the free thio acidor when other conditions are employed which will cause the liberation ofthe free acid from the salt.

Basic ingredients, such as zinc oxide and lime,

exert a very marked retarding action and should be avoided. The basicmaterials which will seriously retard the action of our compounds arethose which are sufliciently active under the conditions employed toreact with the thio acids or sulfides to form salts of the thio acids.When a thio acid is employed, sufilcient basic material to completelyneutralize the acid and form a salt therewith will substantiallycompletely prevent the action of the acid.

Further, some accelerators and antioxidants exert a retarding action onour compounds tovarious degrees and any substantial amounts of them inexcess of 3% should be avoided durin the plasticizing period. In thecase or the basic accelerators and antioxidants which are 'suflicientlyactive to react with the thio acids or sulfides, the rule as to basicmaterials will apply. Sulfur containing accelerators appear to act in amanner similar to free sulfur.

By the term sulfid we intend to include the polysulfides, which readilylose the excess sulfur without imparting sufilcient free sulfur to therubber system to prevent the plasticizing action. 5

Generally,'our compounds will be employed in proportions of about 0.05to about 2%, based on the rubber. Larger or smaller amounts may beemployed if desired, but without advantage. Pre rably, we employ fromabout .2% to about 55 1% of the plasticizing agent, based on the rubber.

Our compounds will be effective at room temperature. However, they willbe more efiective at temperatures above about 50 C. Preferably,temperatures of from about 100 C. to about 60 130 C. will be employed.In most cases,-temperatures above 130 C. will not be required. althoughany temperature may be employed at which it is safe toheatrubber alone.5 In order to more clearly illustrate our invention, the preferred modesof carrying the same into efiect and the advantageous results to beobtained thereby, the following examples are given. In these examples,the mixing was conducted in a steam jacketed enclosed mixer. Theplasticity of the rubber was determined with pendulum actuated parallelplate plastometer described in Industrial and Engineering Chemistry,analytical edition, 8 305 (1936). The plasticity number is proportionalto the work required to deform the rubber, while the recovery is thethickness in millimeters recovered by the rubber after compression. Therecovery figure appears -to be of the most significance as indicatingthe degree of softness of the rubber.

EXAMPLE 1 Table I Per- Plasticity cent Materiel added number Nona Thio-iuroic acid 2- hydroxy-a-thioma hthoic acid Anthraquinone-i -to-carboxylic acid.--

. Exams: 2 This example illustrates the retarding action of sulfur andzinc oxide. The procedure of Example 1 was followed, but the zinc oxideand sulfur, when employed, were added one-half minute before theplasticizing agent. In the following Table 11 P denotes plasticitynumber, and R denotes recovery.

FEW-P999999 ssasszasassa It will be'noted that'in this example, the zincthio-benzoate was eflective in the absence of 70 added sulfur or zincoxide. However, the rubber contained sufiicient acidic compoundsnaturally present therein to release the free thio-benzoic acid underthe condltionseinployed. I

The compounds 01' our invention are generally 76 well known, as are alsothemethodsfor p'repar :hexahydrobenzoyl sulfide ing them. They can bereadily prepared by treating the acid chloride with a sodium sulfidesuch as sodium sulfhydrate, sodium sulfide or other sodium sulfurcompound. The methods of preparing the compounds are more clearlydescribed in more detail in the literature.

Other thio-carboxylic acids which may be employed for our purpose inaccordance with our in- Y vention are:

thiacetic acid thio-propionic acid thio-butyric acid thio-isobutyricacid thio-valeric acid thio-isovaleric acid thio-hexoic acidthio-caprylic acid thio-lauric acid thio-myristic acid thio-palmiticacid thio-hydroxystearic acid thio-acrylic acid thio-crotonic acidthio-oleic acid thio-linoleic acid thio-ricinoleic acid thio-phenylacetic acid thio-cinnamic acid thio-hydrocinnamic acid 1 thio-phenylacrylic acid thio-naphthenic acids thio-toluic acids thio-xyloic acidsmesitylene thio-carboxylic acids thio-ethylbenzoic acidsmono-thio-phthalic acids hydroxy-methyl-thio-benzoic acidsthio-chlorbenzoic acids thio-hexahydrobenzoic -acids thio-a-naphthoicacids thio-beta-naphthoic acids thio-anthranoic acids thio-phenylbenzoicacids Other sulfides which may beemployed are:

toluyl disulfide xyloyl disulfide mesityloyl sulfide mesityloyldisulfide ethylbenzoyl sulfide ethylbenzoyl disulfide o-carboxyl-benzoylsulfide o-carboxyl-benzoyl disulfide hydro w-methyl ben'zoyl sulfidehydroxy-methyl benzoyl disulfide chlorbenzoyi sulfide chlorbenzoyldisulfide hexahydrobenzoyl disulfide a-naphthoyl sulfide a-naphthoyldisulfide b-naphthoyl sulfide b-naphthoyl disulfide anisoyl sulfideanisoyl disulfide anthranoyi sulfide anthranoyl disulfide phenylbenzoylsulfide phenyl benzoyl disulfide acetyl sulfide acetyl disulfidepropionyl sulfide isobutyryl sulfide fonic groups can usually betolerated.

isobutyryl disulflde valeryl sulfide I naphthenoyl disulfide2-hydroxy-3-naphthoyl sulfide 2-hydroxy-3-naphthoyl disulfide sulfide ofanthraquinone-2-thio-carboxylic acid disulfide ofanthraquinone-2-thio-carboxylic acid The various homologues and isomersof the compounds mentioned will also be effective. The radicle R in theformula may be varied within wide limits, since the activityof the thioacid group is so great that the introduction of such substituents asnitro or sul- The application of our compounds to rubber .may be variedin many ways. The rubber may be dissolved in a solvent and the solutiontreated with. a plasticizing agent of our invention, especially at anelevated temperature, to produce rubber cements of low viscosity. Also,the plasticizing agent may be added to the rubber before solution in thesolvent. The preparation of cements of low viscosity may be mostadvantageously effected by dissolving rubber in a solvent w We claim:

containing the plasticiz ing agent. The plasticizing agents may also beincorporated in the rubber by addition to latex before coagulation.

While we have disclosed the incorporation of resistance of the rubber toflow over that which the rubber would have if subjected to the same ourplasticizing agents with rubber in a heated internal mixer,- other typesof apparatus may be employed, such as an ordinary rubber mill, Banburytype of mixer or a Gordon type plasticator. These apparatus will alsooperate more eificiently at the elevated temperatures.

While we have disclosed the preferred embodiments of our invention andthe preferred modes of carrying the same into effect, it will bereadily.

apparent to. those skilled in the art that many changes and variationsmaybe made therein strued as broadly as is permissible in view of theprior art.

1. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof a monothio-carboxylic acid in which the sulfur is in the carboxylgroup, in the absence of more than about 3% of sulfur and in the absenceof sufiicient basic material to neutralize the action of the compound,for a sumcient length of time for said thio-carboxylic acid compound toeffect a marked decrease in the resistance of the rubber to flow overthat which the rubber would have if subjected to the same conditions inthe absence of said thio-carboxylic acid compound.

2. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof an aromatic monothio-carboxylic acid in which pound to efiect amarked decrease in the resistance of the rubber to flow over that whichthe compound.

3. The method of decreasing the resistance to fiow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof a compound having the formula in which R, represents a hydrocarbonradicle, in the absence of more than about 3% of sulfur and in theabsence of sufiicient basic material to in which R representsan aromatichydrocarbon radicle, inthe absence of more than about 3% ofsulfur and inthe absence of suificient basic material to neutralize the action of thecom pound, for 'a sufiicient length of time for said compound to efiecta marked decrease in the conditions in the absence of said compound.

5. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof an aromatic monothio-carboxylic acid containing a single benzene ringand in which the sulfur of the acid is in the carboxyl group, in theabsence of more than about 3% of sulfur and in the absence of sufficientbasic material to neutralize the actionof the compound, for a suffrcientlength of time for said thio-carboxylic acid compound to effect a markeddecrease in the resistance of the rubber to flow over that which therubber would have if subjected'to the same conditions in the absence ofsaid thio-carboxylic acid compound.

6. The method of decreasingtheresistance to flow of unvulcanized rubberwhich comprises suba compound having the formula in which R represents ahydrocarbon radicle of the benzene series, in the absence of more thanabout 3% of sulfur and in the absence of sumcient basic material toneutralize the action of the compound, for a suflicient length of timefor said compound-to effect a marked decrease in the resistance of therubber to flow over that which the rubber would have if subjected to thesame conditions in the absence of said compound.

'7. Rubber obtained by subjecting unvulcanized rubber to the action of amonothio-carboxylic acid in which the sulfur is in the carboxyl group,in the absence of more than about 3% of sulfur and in the absence ofsuflicient basic material to neutralize the action of the compound, fora sufficient length of time for said thio-carboxylic acid compound toeffect a marked decrease in the resistance of the rubber to flow overthat which the rubberwould have if subjected to the same conditions inthe absence of said thio-carboxylic acid compound.

8. Rubber obtained by subjecting unvulcanized rubber to the action of anaromatic monothiocarboxylic acid in which the sulfur is in the carboxylgroup, in the absence of more than about 3% of sulfur and in the absenceof sufficient basic material to neutralize the action of the compound,for a sufficient length of time for said thio-carboxylic acid compoundto effect a marked decrease in the resistance of the rubber to flow overthat which the rubber would have if subjected to the same conditions inthe absence of said-thio-carboxylic acid compound.

9. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof a thio-carboxylic acid compound having the for-' mula R-g-s-x whereinR. represents an organic radicle and X represents a member of the groupconsisting of hydrogen and groups of the formulae wherein R. representsan organic group, both R. and B. being connected to the carboxyl carbonsdirectly by carbon to carbon linkage, in theabsence of more than about3% of sulfur and in the absence of suflicient basic material toneutralize the action of the compound, for a stiflici'ent' length oftime for said thio-carboxylic acid compound to effect a marked decreasein the resistance of the rubber to flow over that which the rubber wouldhave if subjected to the same conditions in the absence of saidthio-carboxyli acid compound. a

10. The method of decreasing the-resistance to flow of unvulcanizedrubber which comprises sub- .jecting the unvulcanized rubber to theaction of a thio-carboxyiic acid compound having the formula Irepresents a member of the group consisting of hydrogen and groups ofthe formulae wherein R represents an organic group, both R and R beingconnected to the carboxyl carbons directly by carbon to carbon linkage,in the absence of compounding ingredients, for a sumcient length of timefor said thio-carboxylic acid compound to effect a marked decrease inthe resistance of the rubber to flow over that which the rubber wouldhave if subjected to the same conditions in the absence of said thio-carboxylic acid compound.

11. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof a thio-carboxylic acid compound having the formula v 0 a- -sr- -n'wherein n represents an integer of 1 to 2 and R and R represent organicradicles connected to the carboxyl carbons by carbon to carbon linkoverthat which the rubber would have if subjected to the same conditions inthe absence of said thio-carboxylic acid compound.

12. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber tothe actionof a thio-carboxylic acid compound havingthe formula Eli-sis wherein nrepresents an integer of 1 to 2 and R. and R. represent aromaticradicles connected to the carbonyl carbons by carbon to carbon linkages,in the absence of more than about 3% of sulfur and in-the absence ofsuflicient basic material to neutralize the action of the comp0und,.

for a suflicient length of time for said thio-carboxylic acid compoundto effect a marked de-- that which the rubber would have if subjected tothe same conditions in the absence of said thio-carboxylic acidcompound. I

13. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof a thio-carboxylicacidcompound having the formula 0 4 atwherein R andR represent hydrocarbon radicles, in the absence of more than about 3%of sulfur and in the absence of suflicient basic-material to neutralizethe action ofthe compound, for a sufficient length of time for saidthio-carboxylic acid compound to effect a marked decrease in theresistance of the rubber to flow over that which the rubber would haveif subjected to the same conditions in the absence of saidthio-carboxylic acid compound.) I

14. The method of decreasing the resistance -to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof a thio carboxylic acid compound having the formula I II n-c-s-c-R'wherein R and R each represent a single benzene ring, in the absence ofmore than about 3% of sulfur and in the absence of sufllcientbasicmaterial to neutralize the action of the compound,

for a; suihcient length of time for said thin-car boxylic acid compoundto eifect a marked decrease in the resistance of the rubber to flow overthat which the rubber would have if subjected to the same conditions inthe absence of said thio-carboxylic acid compound.

15. The method of decreasing the resistance the rubber to flow over thatwhich the rubber would have if subjected to the same conditions in theabsence of said thio-carboxylic acid compound. I a

16. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof thio-benzoic acid, in the absence of more than about 3% of sulfur andin the absence of 'suflicient basicmaterial to neutralize the action ofthe thio-benzoic acid, for a sufllcient length of time for saidthio-benzoic acid to effect a marked decrease in the resistance of therubber to flow over that which the rubber would have if subjected to thesame conditions in the absence of said thio-benzoic acid.

17. Rubber obtained by subjecting 'unvulcanized rubber to the action ofa thio-carboxylic acid compound having the formula I! -ncsx wherein R.represents an organic radicle. and X represents a member of the groupconsisting of hydrogen and. groups of the formulae wherein R representsan organic group, both R and B being connected to the carboxylic carbonsdirectly by a carbon to carbon linkage, in-

a Rina,

wherein n represents an integer of 1 to 2 and R and R represent aromaticradicles connected to the carboxyl carbons by carbon to carbon linkages,in the absence of more than about 3% of sulfur and in the absence of'sufflcient basic material'to neutralize the action of thethio-carboxylic acid compound,-for a suilicient length of time for saidthio-carboxylic compound to effect a marked decrease in the resistanceof the rubber to now over that which the rubber would have if subjectedto the same conditions in the absence'of said thio-carboxylic acidcompound.

19. Rubber obtained by subjecting unvulcan-. ized rubber to the actionof a thio-carboxylic acid compound having the formula II II RCSG-R'wherein R. and R represent hydrocarbon radicles, in the absence of morethan about 3% of sulfur and in the absence of sufiicient basic materialto neutralize the action of the'thio-carboxylic acid compound, for asuificientlength of time for said thio-carboxylic compound to effect I amarked decrease in the resistance of the rubber to flow over that whichthe rubber would have if subjected to the same conditions in the absenceof said thio-carboxylic acid compound.

20. Rubber obtained by subjecting unvulcanized rubber to the action of athio-carboxylic acid compound having the formula 0 4| II R- SCR' whereinR and R each represents a hydrocarbon radicle of the benzene series, inthe absence of more than about 3% of sulfur and in the absence ofsufiicient basic material to neutralize the action of thethio-carboxylic acid compound, for a sufflcient length of time for saidthio-carboxylic compound to eflfecta marked decrease in the resistanceof the' rubber to flow over that which the rubber would have ifsubjected to' the same conditions in absence of said thio-carboxylicacid compound.

21. Rubber obtained by subjecting unvulcanized rubber to the action ofthio-benzoic acid, in

the absence of more than about 3% of sulfur and in the absence ofsufficient basic material to neutralize theaction of the thio-benzoicacid, for a sufiicient length of time for said thio-benzoic acid toeifeot a marked decrease in the resistance of the rubber to flow overthat which the rubber would have if subjected to the same conditions inthe absence of said thio-benzoic acid. I

22. The method of decreasing the resistance to flow of unvulcanizedrubber which comprises subjecting the unvulcanized rubber to the actionof thio-anisic acid, in the absence of more than about 3% of sulfur andin the absence of suificient basic material to neutralize the action ofthe thio-anisic acid, for a sufficient length of time for saidthioanisic acid to effect a marked decrease in the resistance of therubber to flow over that which.

of benzoyl sulfide, in the absence of more than about 3% of sulfur andin the absence of suificient basic material to neutralize the action ofthe benzoyl sulfide, for a suflicient length of time for said benzoylsulfide to effect a marked decrease in the resistance of the rubber toflow over and in the absence of sufiicient basic material to neutralizethe action of the thio-anisic acid, for a sufficient length of time forsaid thio-anisic acid to effect a marked decrease in the resistance ofthe rubber to flow over that which the rubber would have if subjected tothe same conditions in the absence of said thio-anisic acid.

.25. Rubber obtained by subjecting unvulcam ized rubber to the action ofbenzoyl sulfide, in the absence of more than about 3% of sulfur and inthe absence of suflicient basic material to neutralize the action of thebenzoyl sulfide, for a sufficient length of time for said benzoylsulfide to effect a. marked decrease in the resistance of the rubber toflow over that which the rubber would.

have i! subjected to the same conditions in the absence of saidbenzoylsulfide.

' CARROLL commas SMITH.

